Study On Tunnel Stability And Reinforcing Mechanism Of Bolts And Shotcrete Based On Rock Fracture Mechanics

Stability of rock mass is one of the most important problems in rock engineering. Itaffects the engineering progress, tunneling and supporting cost, and may concern the safetyof the constructors. Because of the complexity rock mass and that theory falls behind thepractice, the problem has not solved very well yet. With the rapid development of rockengineering in our country, the theories can not satisfy the need of engineering more andmore. Rock mass is a kind of structural one which is composed by rock blocks anddiscontinuities. The failure of rock mass is usually the result of the growth and coalescenceof the discontinuities. The distribution and state of the discontinuities and their mechanicalcharacteristic has much difference. Therefore, the study of the influence of thediscontinuities on the rock mechanics is a focus and difficult problem. Bolts and shotcretesupporting is one of the widely applied methods in underground rock engineering. But theirreinforcing mechanism for the rock mass is not very clear and needs to be further studied.Rock fracture mechanics study the extension pattern of the discontinuities growth andcoalescence, so it can be used to understand the failure process of the surrounding rock massdeeply. Based the viewpoints of rock fracture mechanics, the failure modes of thesurrounding rock mass of the tunnels are discussed, and the reinforcing mechanism of thebolts and shotcrete is analyzed. The main results discussed in this thesis are as follows: (1) According to the loading characteristic of the surrounding rock mass of the tunnels,the growth and coalescence modes of the discontinuities are analyzed systematically, andthe dominance inclination angle of discontinuities in the rock mass is discussed. Thegrowing and coalescing characteristic of the compression-shear or tensile-shear mixed modein rock mass is studied, too. (2) Taking the tunnel of the circular cross section as the example, the loadingcharacteristic of micro-discontinuities and surface discontinuities surrounding the tunnelsand their fracture modes are analyzed. Based on the theory of fracture mechanics, theformulations of dimension and depth of the disturbed domain are obtained. The results showthat the dimension of disturbed domains is influenced by the cross section of tunnel, in-situstress, fracture toughness of rock, spaces between the discontinuities, length and shearingcapacity of the discontinuities. (3) By theoretical and numerical methods, the resisting function of the shotcerte to thestructural plane growth is studied. Further more, the failure mode and treatment methods of Vthe concrete supporting layer are analyzed by the fracture theory of plates. The results implythat the resisting effects of the shotcrete on the inner discontinuities are less influence thanthat on the surface planes. So, the main function of the shotcrete is used to resist the growthof surface planes. The variety of the shotcrete thickness has little influence on the resistingcapability of the discontinuities. However, the thicker the shotcrete is, the stronger to theexpanding stress of surrounding rock mass will be. The failure zone in the shotcrete can berestricted by bolts. The smaller the distance of the bolts is, the less of the failure zone willbe. (4) The growing modes of the surface discontinuities under compressive load and thefunction of the bolts are studied by the experimental methods. At the same time, thenumerical analysis is used to verify the results. The results demonstrate that there are somewing cracks and secondary cracks at the tip of the surface discontinuities. With the increaseof the inclination angle between the structural plane and the surface, the propagationbecomes more easily. The secondary cracks may be more popular for the case of the smallinclination angles. The role of bolts is to improve the performance of the structural mass,weaken the stress intensity at the c...